Summer 2019
®
As you can see from Table 1, we aren’t just talking about inert chemicals. Some of these herbicides have the potential to be highly toxic to aquatic life alone – not to mention the potential secondary affects from changes to the food chain, water quality, etc.
The following is a brief run down of each of the herbicides, their intended purpose and a few notes regarding their toxicity profiles.
GLYPHOSATE
Glyphosate is an herbicide applied to the leaves of plants to kill both broadleaf plants and grasses.
Glyphosate is a non-selective herbicide, meaning it will kill most plants. It prevents the plants from making certain proteins that are needed for plant growth.
Glyphosate is a difficult one to evaluate – while there is considerable data and research dedicated to the safety and use of glyphosate, there is also considerable data available on the toxicity, and potential carcinogenicity and dangers to humans.
Pure glyphosate is low in toxicity to fish and wildlife, but some products containing glyphosate may be toxic because of the other ingredients in them. Formulated glyphosate products range from moderately toxic to practically non-toxic to freshwater fish.
2,4-D
2,4-Dichlorophenoxyacetic acid (2,4-D) is a selective herbicide used to control broadleaf weeds. 2,4-D is the first successful selective herbicide developed (1942, marketed by 1944).
It is the third most widely used herbicide in the United States and Canada and the most widely used herbicide worldwide. 2,4-D controls plant growth by triggering reactions in plant cells that affect critical cell functions and cell growth, leading to plant death.
It is moderately-to-highly toxic to fish and varies widely depending on chemical form, with esters being the most toxic. The greater toxicity generally of the esters in fish
is likely due to the greater absorption rates of the esters through the gills, where they are hydrolyzed to the acid form.
IMAZAMOX
Imazamox is a systemic herbicide that moves throughout the plant tissue and prevents plants from producing a necessary enzyme, acetolactate synthase (ALS), which is not found in animals. Susceptible plants will stop growing soon after treatment, but plant death and decomposition will occur over several weeks.
Laboratory tests using rainbow trout, bluegill, and water fleas indicate that Imazamox is not toxic to these species at label application rates; however, LC50 toxicity is comparable to glyphosate and can be lethal at higher concentrations.
FLURIDONE
Fluridone interferes with the synthesis of RNA, proteins, and carotenoid pigments and thereby affects photosynthesis of the targeted plants.
Specifically, Fluridone inhibits the formation of carotene, a plant pigment, causing the rapid degradation of chlorophyll by sunlight, preventing the formation of carbohydrates necessary to sustain the plant. Fluridone is slightly to moderately toxic to fish.
The alarming risk of Fluridone is its sub-lethal and chronic affects. Two studies are available on the longer-term effects of Fluridone to fish: a 60-day growth and survival study in channel catfish and a life-cycle study in fathead minnows. In the catfish study, growth was significantly reduced at 1 mg/L (a very low concentration).
In the fathead minnow study, no effects on growth, survival, or hatching were noted at 0.48 mg/L. No eggs hatched, however, at the two next higher concentrations, 0.96 and 1.9 mg/L. As with some of the acute toxicity studies, this pronounced difference between egg hatching in the 0.48 mg/L group (no effect) and the 0.96 mg/L group (no hatching) suggests a relatively steep dose-response relationship.
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